Every single cell in a multicellular organism can in principle trace its "ancestry" back to the zygote through a continuous chain of cell divisions. How many divisions have occurred in a typical cell in an adult human body since the zygote was fertilized?
Obviously the answer depends on the organism's age and on the type of cell (as different types of cells divide at different rates). To be concrete, what is the average number of cell generations from the zygote across all the cells in, say, a typical 25-year-old human?
(Or any other age, if that's more convenient to estimate. I'd also be curious about the approximate minimum and maximum number of divisions, and which types of cells they occur in. If it's too difficult to estimate these statistics over the whole organism, I'd be interested to know if there are any specific types of specialized cells for which it's possible to estimate the "generation number".)
To be clear, I'm not asking about the maximum number of times that a cell can divide. I'm asking about the average number of times it has divided at a given point in time.